And, I believe, all of them were derived from the Hebrew word for 7 -- sheba/sheva -- which then became seven (EN), sieben (DE) and, possibly, even семь (RU).
EDIT: I was wrong about the connection between Hebrew and Indo-European numbers - see Gwenci's comments below. This link may also be of interest: http://forward.com/articles/203645/why-the-hebrew-numbers-six-and-seven-sound-almost/
@Gwenci Thanks, I stand corrected.
P.S. Here is an interesting short article about the possible origins of this quite striking similarity: http://forward.com/articles/203645/why-the-hebrew-numbers-six-and-seven-sound-almost/
Actually, it's a bit more complex. "В" can take prepositional even when it refers to time when you use it in the sense of "within":
В этом месяце 31 день = There are 31 dais in this month Мы встретились в прошлом году = We met last year
But: Мы встретились в прошлый четверг (accusative!) = We met last Thursday.
I cannot quite explain why the last two examples use different cases, but it should be prepositional with months and years and accusative with the days of the week. Go figure...